1. Field of the Invention
The present invention relates to a driving technology suited for an image pickup apparatus and the like.
2. Related Background Art
The technical concept of a driving apparatus having the following construction is disclosed in Japanese Patent Application Laid-Open No. 2002-051524. This driving apparatus includes: a magnet formed in a cylindrical shape, its outer peripheral surface being circumferentially divided into a plurality of sections that are alternately magnetized in different polarities; a first bobbin wound by a coil; a second bobbin wound by another coil; a first outer magnetic pole portion excited by the coil wound around the first bobbin and opposed to the outer peripheral surface of the magnet on one end side thereof; a first inner magnetic pole portion having a substantially cylindrical and hollow shape and opposed to an inner peripheral surface of the magnet; and a second outer magnetic pole portion excited by the coil wound around the second bobbin and being opposed to the outer peripheral surface of the magnet on the other end side thereof. The second inner magnetic pole portion has a substantially cylindrical and hollow shape and is opposed to the inner peripheral surface of the magnet. The apparatus also includes a moving unit coupled to the magnet and optical means which has its optical axis in a hollow portion of the first inner magnetic pole portion or in a hollow portion of the second magnetic pole portion and rotates to thereby move the optical means in an optical axis direction.
A positive effect is obtained according to the technical concept of the above-mentioned construction. For example, the driving apparatus has a compact structure in which the coils and the magnet do not occupy a large area on a bottom board. One of the factors behind that effect is that the coils and the magnet are disposed axially.
However, the foregoing construction leads to an increase in the length of the driving apparatus in the optical axis direction and complication of a drive circuit thereof. According to some product specifications, a lens may be driven between two different positions. For instance, the lens may move between a storage position and a service position, or the lens may move between a normal shooting distance position and a macro shooting position. For such cases, a mechanism for moving a lens simply between two different positions has been desired.
To fulfill this desire, the technical concept of a driving apparatus having the following construction is disclosed in Japanese Patent Application Laid-Open No. 2004-048873. This driving apparatus includes: a lens; let-out means; a magnet that is rotatable and whose outer peripheral surface has a cylindrical magnetizing portion that is circumferentially divided so that its divided portions are alternately magnetized in different polarities; a coil disposed in an axial direction of the magnet; a stator having at least one tooth-shaped outer magnetic pole portion and a substantially cylindrical and hollow inner magnetic pole portion, which are respectively opposed to the outer peripheral surface and an inner peripheral surface of the magnetizing portion of the magnet, the stator being excited by the coil; and a lens having its light path in a hollow and cylindrical portion of the inner magnetic pole portion of the stator. The let-out means lets the lens out along an optical axis as the magnet rotates. The tooth-shaped outer magnetic pole portion is opposed to the outer peripheral surface of the magnetizing portion of the magnet opposed thereto within a predetermined angular range, so that the condition −0.3X+0.63>Y is satisfied, where Y represents a ratio of an angle of one magnetized pole of the magnetizing portion of the magnet to the predetermined angle at which the outer magnetic pole portion is opposed to the magnetizing portion of the magnet, and X represents a ratio of a circumferential length of one magnetized pole of the magnetizing portion of the magnet to a radial thickness of the magnet.
Owing to the technical concept of the aforementioned configuration, the central position of the magnetized pole of the magnet is stably held at a position opposed to the center of the tooth of the outer magnetic pole portion when the coil is not energized. The magnet is driven such that the central position of the magnetized pole of the magnet moves among positions opposed to the centers of teeth of the outer magnetic pole portion when the coil is energized. The magnetizing portion of the magnet driven through energization of the coil receives an attraction force acting in a rotational direction due to a magnetic force that is generated in the stator such that the magnetizing portion stays where it is, when the coil is stopped from being energized.
In other words, the lens can be held in its let-out state or its let-in state even during cessation of energization after the lens has been driven once through energization. As a result, a driving apparatus with a simple drive circuit construction and low power consumption can be realized.
However, according to the technical concept disclosed in the above-mentioned Japanese Patent Application Laid-Open No. 2004-048873, a relatively high voltage is required for driving. Therefore, a reduction in the voltage for energizing the coil is demanded for the sake of further power saving. In the following description, art meeting such a demand will be considered.
FIG. 5 shows a simulation result obtained by surveying torque characteristics of a magnet provided in the above-mentioned driving apparatus or the like by means of a magnetic field analysis based on a finite element method. The abscissa axis represents a rotational position of the magnet, and the ordinate axis represents torque acting on the magnet. A line of 0 V (broken line) indicates a cogging torque at the time when a coil is stopped from being energized. FIG. 5 shows how the energization torque rises by raising the voltage for energizing the coil to 3 V and then to 5 V. As for the cogging torque, at points E1 and E2 shown in FIG. 5, when the magnet is about to rotate in a positive direction, it receives a negative force and starts returning to its original position. On the other hand, when the magnet is about to rotate in a reverse direction, it receives a positive force and is returned to its original position. In other words, the magnet assumes a cogging position where it is urged to be stably positioned at the point E1 or E2 due to magnetic force acting between the magnet and the outer magnetic pole portion. A point F in FIG. 5 represents an unstable position. The magnet assumes an unstably balanced state where a rotational force acts on one of the points E1 and E2 located straddling the point F even if the magnet is slightly shifted in phase at the point F. The magnet does not remain stopped at the point F because of vibrations or a change in posture and is stopped at the point E1 or E2, when the coil is not energized. On the assumption that the number of magnetized poles of the magnet is n, cogging stable points such as the points E1 and E2 exist at angular intervals of 360/n . A midpoint between the points E1 and E2 spaced apart from each other becomes the unstable point such as the point F. By being provided with rotation stoppers, the magnet can move within a range between the two stable positions. In FIG. 5, this can be realized by providing the magnet with the rotation stoppers at a position θ1′ (corresponding to the first position) and at a position θ2′ (corresponding to the second position).
In order to hold a lens barrel against gravity or vibrations during cessation of energization at each of the positions, a cogging torque of a certain magnitude or more (whose absolute value is equal to or larger than Cmin shown in FIG. 5) is required. For instance, the lens barrel can be held by setting the movable range of the magnet to a “movable range 1”.
In order to activate the lens barrel against friction or gravity during energization at each of the positions, an energization torque of a certain magnitude or more (whose absolute value is equal to or larger than Tmin shown in FIG. 5) is required. Thus, the voltage of 3 V for energizing the coil is insufficient for activation of the lens barrel when the movable range of the magnet is set to the movable range 1. A higher voltage (energization with +5 V from the first position to the second position, and energization with −5 V from the second position to the first position) is required for activation of the lens barrel. This makes power saving difficult.
In order to activate the lens barrel with a lower voltage, it is conceivable to set the first position and the second position to θ1 and θ2, respectively, that is, to set the movable range of the magnet to a movable range 2. Then, at the first position, Tmin can be exceeded even when the voltage for energizing the coil is 3 V. Consequently, the lens barrel can be activated with a lower voltage from the first position to the second position. However, the cogging torque C1 at this moment is less than the torque Cmin required for holding the lens barrel. Accordingly, the lens barrel cannot be held in position due to the influence of gravity or vibrations during cessation of energization.
Therefore, if a construction in which the force for holding the lens barrel is equal to or larger than the aforementioned torque Cmin is adoptable, the lens barrel needs to be held reliably. In addition, the voltage required for moving the lens barrel from the first position to the second position needs to be lowered. The applicant of the present invention has devised a new driving apparatus of this type. The lens barrel can be activated from the second position to the first position by applying the same voltage (−5 V) as in the conventional art, even when the movable range of the magnet is set to the “movable range 2”. Since the cogging torque is also larger than Cmin, the lens barrel can be held in position even when it is subjected to the influence of gravity or vibrations during cessation of energization at the second position.